Industrial blowers are well known in the industry for efficiently generating large quantities of air flow. This air flow is used for generating an air flow in industrial processes or generating a suction or vacuum force. Such applications include, but are not limited to: air-assisted breathing; air-assisted inflation or support for material handling, paper processing, floatation tables; air-assisted vacuum pick-up or hold-down; air and gas sampling, boosting or circulating; electronic cooling; gas, vapor and fume recovery, venting and treatment; solid material transportation, separation and collection; parts blow-off and drying; solution and media agitation, and aeration. These blowers, and machinery in general, create lots of noise which are considered by many to be a form of pollution. Prolonged exposure to high levels of noise can damage an individual's hearing and is considered to be generally uncomfortable at lower levels. It will also be appreciated that the noise generated by these machines contribute to inefficiency in the operation of the machine and lead to premature wear and a waste of energy.
Prior art blower assemblies typically employ an electric motor that rotates a shaft that is connected to an impeller or fan. The impeller is contained within a blower housing that forms an enclosed annular chamber. Fluidly connected with the annular chamber is an inlet port and an outlet port. As the motor is energized, air is drawn in the inlet port by the impeller, pressurized and then expelled out the outlet port. In particular, the impeller blades pass the inlet port and draw air or other gases into the blower. The impeller blades then, by centrifugal action, accelerate the air outward and forward. Depending upon the construction of the impeller and the annular chamber a “regenerative” principle may take effect so that the air is turned back by the annular shaped housing to the base of the following blades where it is again hurled outward. Each “regeneration” imparts more pressure to the air. When the air reaches a “stripper” section at the outlet, wherein the stripper is the part of the housing located between the inlet and the outlet in which the annulus is reduced in size to fit closely to the sides and tips of the impeller blades, the air is “stripped” from the impeller and diverted out of the blower. The pressures or vacuums generated by the one or multiple spinning, non-contacting impellers are equal to those obtained by many larger multi-stage or positive displacement blowers.
Although these blowers are effective in generating a desired pressure or air flow it will be appreciated that a significant amount of noise is also generated. It is believed that the noise is primarily generated by the impeller blades passing by the edges of the housing and the sharp airflow turns encountered in routing the air through the inlet, the annular chamber and the output port. A significant noise source is sometimes referred to as a “blade passing frequency” which is generated by the impeller blades passing a fixed point such as the housing or stripper section. This frequency may be estimated by the number of impeller blades, times the impeller's revolutions per minute, divided by 60 (seconds per minute). This frequency varies with blower speed and environmental changes to the speed of sound. Additional features of the impeller, such as strength ribs, may generate additional noise components. Harmonics of these prime noise generators also contribute to the overall noise of the blower. It is known to provide internal baffles and noise absorbing foam at the inlets and outlets but these are not directly associated with the source of the noise. Therefore, there is a need in the art for a more direct sound absorbing or noise minimizing feature associated with the source of the noise. And there is also a need to improve airflow properties through the blower so as to reduce turbulence so as to further reduce generated noise.